single phase motor proplem

[Jamie9241]

Hello,
was called to a school today to investigate a fault on a heater unit. It is realatively simple set up, it consists of a water heated heatextchange and a single phase capacitive start motor, with a variable resistor set at 50 ohm in series with the phase conductor, i presume so the speed can be alterd depending on the size of room it is insalled in. also there is variouse temp sensors.

On inspection of the unit I found that the variable resistor had burnt out and gone open ciruit. so we aquired a new one and replace the resistor and tested the unit. we further then found that the motor had failed.

In testing the motor i tested for any earth fault with IR tester and got high good reading, I tested the resistance of the start winding and got an acceptabe low result, I tested the run winding and got open circuit indicating that thier was a breack in the winding some were,

Why would the variable resistor failing cause the winding in the motor to go open circuit.
I have had the motor sent to be re-wound. But i would have liked to know why the resistor failing would cause damage to the motor.

Thanx for any advise

 

[davidbeach]

Maybe it didn't. Could be that both are the result of the same external event, such as a transient overvoltage. Could be that you have cause and effect reversed and motor failure leading to open winding damaged resistor.

 

[roydm]

Perhaps a short in the winding blew the resistor then cleared itself.
Roy

 

[jraef]

I don't know of any type of 1 phase cap-start motor that can be controlled by dropping the voltage going to it with a resistor. It will burn out.

It could be that the system was originally designed to be used with a split-phase or a shaded pole motor, and at some point the motor failed and was replaced by the cap-start one, but that would be an inappropriate replacement. Unless there is some kind of special motor out there that I am unaware of.


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[Jamie9241]

I'm not sure of the function of the resistor it is a porcelain tube with a tungsten type of wire coiled around it. On the tube Their is a common connection to connect the feed then their is a second connection witch clamps round the porcelain tube with can be moved to any position on the tube to increase or decrease the tap of resistance, when i checked another working unit i measured the resistance and it was set to 50ohm.

What i think may have happen is that a fault has occurred in the motor causing a overload, Because of the 50 ohm resistor this would have kept the over load to low to blow the fuse in the spur unit and cause the actual resistor to heat up like a heater element and burn out. That's wot i think but i may be wrong, These heaters are pretty old unit probable installed in the 60s.

Thanx for any advise. I find this site very usfull and educational. As a lot of things mention on here i have come up agaist in the past in my job but never had a proper explanition as to the root cause of the proplem or failre.

 

[Tech805]

Not being familiar with the controller of this motor, I would guess the system tried several times to start the motor. And, with an open run winding and a high current start winding the resistor over heated and ultimately failed.

I've worked on a wide variety of motors and I can't say I've ever seen one with a resistor...